High quality digital cameras have to a large extent replaced film cameras. However, like film cameras, with digital cameras much attention has been placed by the camera industry on the size and quality of lenses which are used on the camera. Individuals seeking to take quality photographs are often encouraged to invest in large bulky and often costly lenses for a variety of reasons. Among the reasons for using large aperture lenses is their ability to capture a large amount of light in a given time period as compared to smaller aperture lenses. Telephoto lenses tend to be large not only because of their large apertures but also because of their long focal lengths. Generally, the longer the focal length, the larger the lens. A long focal length gives the photographer the ability to take pictures from far away.
In the quest for high quality photos, the amount of light which can be captured is often important to the final image quality. Having a large aperture lens allows a large amount of light to be captured allowing for shorter exposure times than would be required to capture the same amount of light using a small lens. The use of short exposure times can reduce blurriness especially with regard to images with motion. The ability to capture large amounts of light can also facilitate the taking of quality images even in low light conditions. In addition, using a large aperture lens makes it possible to have artistic effects such as small depth of field for portrait photography.
Large lenses sometimes also offer the opportunity to support mechanical zoom features that allow a user to optically zoom in or out and/or to alter the focal length of the lens which is important for framing a scene without the need to move closer or further from the subject.
While large lenses have many advantages with regard to the ability to capture relatively large amounts of light compared to smaller lenses, support large zoom ranges, and often allow for good control over focus, there are many disadvantages to using large lenses.
Large lenses tend to be heavy requiring relatively strong and often large support structures to keep the various lenses of a camera assembly in alignment. The heavy weight of large lenses makes cameras with such lenses difficult and bulky to transport. Furthermore, cameras with large lenses often need a tripod or other support to be used for extended periods of time given that the sheer weight of a camera with a large lens can become tiresome for an individual to hold in a short amount of time.
In addition to weight and size drawbacks, large lenses also have the disadvantage of being costly. This is because of, among other things, the difficulty in manufacturing large high quality optics and packaging them in a manner in which they will maintain proper alignment over a period of time which may reflect the many years of use a camera lenses is expected to provide.
A great deal of effort has been directed in the camera industry to supporting the use of large camera lenses and packaging them in a way that allows different lenses to be used in an interchangeable manner on a camera body. However, for the vast majority of camera users, the drawbacks to cameras with large lenses means that camera users tend not to use large lenses with such lenses often being left to professionals and/or photo enthusiasts willing to incur the expense and trouble of buying and using large lenses.
In fact, many camera owners who own cameras with large high quality lenses often find themselves taking pictures with small pocket size cameras, often integrated into other devices such as their cell phones, personal digital assistants or the like, simply because they are more convenient to carry. For example, cell phone mounted cameras are often more readily available for use when an unexpected photo opportunity arises or in the case of a general family outing where carrying large bulky camera equipment may be uncomfortable or undesirable.
To frame a given scene from a given point, the focal length (hence size) of the lens depends on the size (area) of the image sensor. The smaller the image sensor, the smaller the focal length and the smaller the lens required. With advances in sensor technology, it is now possible to make small sensors, e.g., 5×7 mm2 sensors, with relatively high pixel count, e.g., 8 megapixels. This has enabled the embedding of relatively high resolution cameras in small devices such as cell phones. The small sensor size (compared to larger cameras such as changeable lens single-lens reflex (SRL) cameras) enables small focal length lenses which are much smaller and lighter than larger focal length lenses required for cameras with larger sensors.
Cell phone mounted cameras and other pocket sized digital cameras sometimes rely on a fixed focal length lens which is also sometimes referred to as a focus-free lens. With such lenses the focus is set at the time of manufacture, and remains fixed. Rather than having a method of determining the correct focusing distance and setting the lens to that focal point, a small aperture fixed-focus lens relies on a large depth of field which is sufficient to produce acceptably sharp images. Many cameras, including those found on most cell phones, with focus free lenses also have relatively small apertures which provide a relatively large depth of field. There are also some high end cell phones that use auto focus cameras.
For a lens of a digital camera to be useful, it needs to be paired with a device which detects the light passing through the lens and converts it to pixel (picture element) values. A megapixel (MP or Mpx) is one million pixels. The term is often used to indicate the number of pixels in an image or to express the number of image sensor elements of a digital camera where each sensor element normally corresponds to one pixel. Multi-color pixels normally include one pixel value for each of the red, green, and blue pixel components.
In digital cameras, the photosensitive electronics used as the light sensing device is often either a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) image sensor, comprising a large number of single sensor elements, each of which records a measured intensity level.
In many digital cameras, the sensor array is covered with a patterned color filter mosaic having red, green, and blue regions in an arrangement. A Bayer filter mosaic is one well known a color filter array (CFA) for arranging RGB color filters on a square grid of photo sensors. Its particular arrangement of color filters is used in many digital image sensors. In such a filter based approach to capturing a color image, each sensor element can record the intensity of a single primary color of light. The camera then will normally interpolate the color information of neighboring sensor elements, through a process sometimes called demosaicing, to create the final image. The sensor elements in a sensor array using a color filter are often called “pixels”, even though they only record 1 channel (only red, or green, or blue) of the final color image due to the filter used over the sensor element.
While a filter arrangement over a sensor array can be used to allow different sensor elements to capture different colors of light thus allowing a single sensor to capture a color image, the need to carefully align the filter area with individual pixel size sensor elements complicates the manufacture of sensor arrays as compared to arrays which do not require the use of a multi-color filter array. Furthermore, the fact that multiple colors of light need to pass through the camera lenses to reach the sensor so that the sensor can measure multiple different colors of light means that the lens can not be optimized for a single color of light and that some chromatic aberration is likely to result. Chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light sometimes referred to as the dispersion of the lens. While small focal length lenses paired with relatively high resolution sensors have achieved widespread commercial success in cell phones and pocket cameras, they often leave their owners longing for better picture quality, e.g., picture quality that can only be achieved with a larger pixel area and a larger lens opening to collect more light.
Smaller sensors require smaller focal length lenses (hence smaller lenses) to frame the same scene from the same point. Availability of high pixel count small sensors means that a smaller lens can be used. However, there are a few disadvantages to using smaller sensors and lenses. First, the small pixel size limits the dynamic range of the sensor as only a small amount of light can saturate the sensor. Second, small lenses collect less total light which can result in grainy pictures. Third, small lenses have small maximum apertures which make artistic effects like small depth of field for portrait pictures not possible.
In the case of a single lens being used to capture color images, there are often problems associated with the use of a color filter mask between the lens and the image sensor. This is because the multi-color grid mask needs to be carefully aligned with the image sensor elements for proper color separation. While the use of a multi-color grid mask has the benefit of allowing a single lens and image sensor to capture color images, the number of sensor elements per image area is reduced on a per color basis as compared to the case where the image sensor is dedicated to capturing light corresponding to a single color. For example, if an 18 MP sensor is used to evenly capture R, G and B light, on a per color basis each color would be captured at only a 6 Mpel resolution. In addition to resulting in a per color pixel resolution that is lower than the overall pixel resolution of the sensor, the cost of camera modules using a multi-color mask tends to be more expensive then the cost of monochrome, i.e., single color, camera modules using monochrome lens which do not require a grid like filter. From the above discussion it should be appreciated that the use of camera modules with multi-color grid filters between a camera lens and photo sensor have several disadvantages.
From the above, it should be appreciated that there is a need for improved methods and apparatus for capturing and/or generating color images which avoid or reduce the need for image sensors with multi-color filters.